Selectivity in metal-carbon bond protonolysis in p-tolyl-(or methyl)-cycloplatinated(ii) complexes: Kinetics and mechanism of the uncatalyzed isomerization of the resulting Pt(ii) products

Article abstract of DOI:10.1039/c3dt51339d

Reaction of each of the known starting complexes [PtR(C^N)(SMe ₂)], 1, in which R = Me or p-MeC₆H₄ and C^N is either ppy (deprotonated 2-phenylpyridine) or bhq (deprotonated benzo[h]quinoline), with one equivalent of CF₃CO₂H, gave the complexes [Pt(C^N)(CF₃CO₂)(SMe₂)], 3 (C^N = ppy, 3a; bhq, 3b). The bis-chelate complexes [Pt(C^N)(P^P)](CF ₃CO₂), 4, were obtained by reaction of complexes 3 with one equivalent of either of the P^P bisphosphine reagents, dppf = 1,1′-bis(diphenylphosphino)ferrocene or dppe = bis(diphenylphosphino) ethane. Complexes 4 were alternatively made by reaction of the complexes [PtMe(κ¹C-C^N)(P^P)], 2, with one equivalent of CF ₃CO₂H. When the complex 3b was reacted with 0.5 equivalents of dppe, 0.5 equivalents of the related bis-chelate product, 4d, formed along with 0.5 equivalents of the unreacted starting complex 3b. In contrast, when the complex 3b was reacted with 0.5 equivalents of dppf, then the dimeric complex [Pt₂(bhq)₂(CF₃CO ₂)₂(μ-dppf)], 5, formed in pure form. In all the above-mentioned acid reactions, the M-R bond rather than the M-C bond of the cycloplatinated complex is cleaved. When the PPh₃ analogues of complexes 1, i.e. the complexes [PtR(C^N)(PPh₃)], 6, in which C^N is ppy or tpy = deprotonated 2-p-tolylpyridine, were reacted with one equivalent of CF₃CO₂H, the course of the reaction reversed and the M-C bonds of the cycloplatinated complexes are cleaved rather than the M-R bonds. The latter reaction gave [PtR(κ¹N-HC^N)(PPh₃) (CF₃CO₂)], as an equilibrium mixture of two isomers 7 and 8. Crystal structures of the typical complexes show a variety of extensive intermolecular hydrogen bonding involving C-H bonds from the different ligands and electronegative atoms (O or F) from the CF₃CO₂ moiety. On the basis of data obtained from kinetic studies (using ¹H NMR spectroscopy), a dissociative mechanism is proposed for the case of the 7c/8c isomerization process, involving dissociation of the κ¹N-Htpy neutral ligand, rather than the alternative route of PPh₃ or CF ₃CO₂ ligand dissociation.

Full text of DOI:10.1039/c3dt51339d

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Selectivity in metal–carbon bond protonolysis in
p-tolyl- (or methyl)-cycloplatinated(II) complexes:
kinetics and mechanism of the uncatalyzed
isomerization of the resulting Pt(II) products†‡
Cite this: Dalton Trans., 2013, 42, 13369
Mohsen Golbon Haghighi,a S. Masoud Nabavizadeh,*a Mehdi Rashidi*a and
Maciej Kubickib
Reaction of each of the known starting complexes [PtR(C^N)(SMe2)], 1, in which R = Me or p-MeC6H4
and C^N is either ppy (deprotonated 2-phenylpyridine) or bhq (deprotonated benzo[h]quinoline), with
one equivalent of CF3CO2H, gave the complexes [Pt(C^N)(CF3CO2)(SMe2)], 3 (C^N = ppy, 3a; bhq, 3b).
The bis-chelate complexes [Pt(C^N)(P^P)](CF3CO2), 4, were obtained by reaction of complexes 3 with one
equivalent of either of the P^P bisphosphine reagents, dppf = 1,1’-bis(diphenylphosphino)ferrocene or
dppe = bis(diphenylphosphino)ethane. Complexes 4 were alternatively made by reaction of the com-
plexes [PtMe(κ1C-C^N)(P^P)], 2, with one equivalent of CF3CO2H. When the complex 3b was reacted with
0.5 equivalents of dppe, 0.5 equivalents of the related bis-chelate product, 4d, formed along with
0.5 equivalents of the unreacted starting complex 3b. In contrast, when the complex 3b was reacted
with 0.5 equivalents of dppf, then the dimeric complex [Pt2(bhq)2(CF3CO2)2(μ-dppf)], 5, formed in pure
form. In all the above-mentioned acid reactions, the M–R bond rather than the M–C bond of the cyclo-
platinated complex is cleaved. When the PPh3 analogues of complexes 1, i.e. the complexes [PtR(C^N)-
(PPh3)], 6, in which C^N is ppy or tpy = deprotonated 2-p-tolylpyridine, were reacted with one equivalent
of CF3CO2H, the course of the reaction reversed and the M–C bonds of the cycloplatinated complexes are
cleaved rather than the M–R bonds. The latter reaction gave [PtR(κ1N-HC^N)(PPh3)(CF3CO2)], as an equili-
brium mixture of two isomers 7 and 8. Crystal structures of the typical complexes show a variety of
extensive intermolecular hydrogen bonding involving C–H bonds from the different ligands and electro-
negative atoms (O or F) from the CF3CO2 moiety. On the basis of data obtained from kinetic studies
(using 1H NMR spectroscopy), a dissociative mechanism is proposed for the case of the 7c/8c
isomerization process, involving dissociation of the κ1N-Htpy neutral ligand, rather than the alternative
route of PPh3 or CF3CO2 ligand dissociation.
Received 22nd May 2013,
Accepted 20th June 2013
DOI: 10.1039/c3dt51339d
www.rsc.org/dalton
luminescence properties.11–21 We have recently reported some
new C,N donor cycloplatinated(II) complexes with bispho-
Introduction
Cyclometallated complexes are of continuing interest due to sphine ligands, such as dppm = bis(diphenylphosphino)-
the possibility of their diverse applications in industrial and methane or dppa = bis(diphenylphosphino)amine, and indi-
medical technologies.1–10 In this regard, platinum complexes cated that they have significant capabilities of being further
have been extensively investigated and indicated to have investigated as anticancer drugs.20,21 In these complexes
medicinal applications due to their cytotoxicity and anionic leaving groups are designed to be situated trans to a C
ligating atom having a high trans influence, allowing the group
to leave easier as compared to one being trans to other types of
ligands.
aDepartment of Chemistry, Faculty of Sciences, Shiraz University, Shiraz 71454, Iran.
E-mail: rashidi@chem.susc.ac.ir, nabavi@chem.susc.ac.ir; Fax: +98 711 6460788;
Tel: +98 711 6460724
Mechanistic aspects of the protonolysis of M–R (R = alkyl
or aryl) bonds, which is considered as the microscopic
reverse reaction of selective activation and functionalization
of saturated alkane C–H bonds, have been extensively
investigated.22–33 The reports in the literature indicate that
usually in alkyl-arylplatinum(II) and methyl-cycloplatinated(II)
bFaculty of Chemistry, Adam Mickiewicz University, Umultowska 89b,
61-614 Poznan, Poland
†Dedicated to Professor Richard J. Puddephatt on the occasion of his 70th
birthday for his great impact in the field of organometallic chemistry.
‡CCDC 919435–919438. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt51339d
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complexes, the Pt–C(alkyl) bond, rather than the Pt–C(aryl) CH2Cl2, 0.1 mmol). Immediately a green solution was formed.
bond, is cleaved in Pt–R protonolysis.34–44 However, there are a The solution was stirred for 1 h, then the solvent was removed
few other reports showing that quite different selectivity in under reduced pressure, and the residue was triturated with
Pt–C bond protonation is observed on changing the spectator diethyl ether (2 × 2 mL). The product as a green solid was
ligands in alkyl-arylplatinum(II), and alkyl- (or aryl)-cycloplati- dried under vacuum. Yield: 47.7 mg, 91%; m.p. = 152–155 °C
nated(II) complexes.45,46
(decomp.). C15H14F3NO2PtS (524.2): calcd C, 34.3; H, 2.69;
We report here the synthesis of a number of new bis-chelate N, 2.67; found: C, 33.9; H, 2.62; N, 3.01%. NMR data in CDCl3:
complexes of the type [Pt(C^N)(P^P)][CF3CO2], 4, in which C^N = 1H NMR: δ = 2.65 (s, 3J(PtH) = 53.2 Hz, SMe2, 6H), aromatic
2
3
ppy or bhq and P^P = dppe or dppf, by the protonolysis of protons: 8.51 (d, J(HH) = 5.8 Hz, J(PtH) = 35.2 Hz, the C–H
some proper p-tolyl- (or methyl)-cycloplatinated(II) precursors. proton adjacent to N of ppy, i.e. H6, 1H), other aromatic
In these reactions, we found that the M–C bond of the aryl or protons of ppy ligand: 7.0–8.0.
methyl is cleaved rather than the related M–C bond of the
The analogous complex [Pt(bhq)(CF3CO2)(SMe2)], 3b, was
cycloplatinated ring. In the protonolysis of related analogous similarly prepared using the related starting complexes [PtMe-
complexes with monodentate phosphine ligand PPh3, i.e. com- (bhq)(SMe2)], 1b, or [Pt(p-MeC6H4)(bhq)(SMe2)], 1d. Yield:
plexes [PtR(C^N)(PPh3)], 6, we observed that, in contrast, the 50.4 mg, 92%; m.p. = 195–198 °C (decomp.). C17H14F3NO2PtS
M–C bond of the cycloplatinated ring, rather than the M–C (548.2): calcd C, 37.2; H, 2.55; N, 2.57; found: C, 37.1; H, 2.46;
1
3
bond of the aryl or even methyl groups, is cleaved. A dissocia- N, 2.85%. NMR data in CDCl3: H NMR: δ = 2.73 (s, J(PtH) =
tive mechanism involving dissociation of the κ1N-Htpy ligand, 54.5 Hz, SMe2, 6H), aromatic protons: 8.73 (d, 2J(HH) = 5.4 Hz,
rather than the alternative route of CF3CO2 ligand dissociation, 3J(PtH) = 35.1 Hz, the C–H proton adjacent to N of bhq, i.e. H6,
is proposed for isomerization of the resulting square-planar 1H), other aromatic protons of bhq ligand: 7.3–8.4.
products obtained in the latter reactions. However, in other
reported systems with a similar geometry,41,45–47 usually a co-
[Pt(ppy)(dppf)][CF3CO2], 4a
ordinated solvent or an anionic ligand leaves and the resulting
T-shape cationic intermediate helps in the isomerization
(a) To a solution of complex [PtMe(κ1C-ppy)(dppf)], 2a,
process.
This was prepared by two methods:
(91.9 mg, 0.1 mmol) in dichloromethane (20 mL) was added a
stock solution of CF3CO2H (781 μL of 1% solution of CF3CO2H
in CH2Cl2, 0.1 mmol). The solution was stirred for 1 h, then
the solvent was removed under reduced pressure, and the
Experimental
residue was triturated with diethyl ether (2 × 2 mL). The
1H NMR spectra were recorded on a Bruker Avance DPX
product as a yellow solid was dried under vacuum.
(b) To a solution of [Pt(ppy)(CF3CO2)(SMe2)], 3a, (52.4 mg,
0.1 mmol) in acetone (20 mL) was added dppf (55.4 mg,
250 MHz spectrometer and 31P NMR spectra were recorded on
a Bruker Avance DRX 500 MHz spectrometer. The operating
frequencies and references, respectively, are shown in paren-
0.1 mmol). The mixture was stirred at room temperature for
theses as follows: 1H (250 MHz, TMS), 31P (202 MHz, 85%
1 h. The solvent was removed and the residue was triturated
H3PO4). Chemical shifts and coupling constants are in ppm
with diethyl ether (2 × 2 mL). The product as a yellow solid was
and Hz, respectively. Microanalysis was performed using a
Thermo Finnigan Flash EA-1112 CHNSO rapid elemental ana-
lyzer. 2-Phenylpyridine, benzo[h]quinoline, 2-p-tolylpyridine,
bis(diphenylphosphino)ethane and 1,1′-bis(diphenylphosphino)-
ferrocene were purchased from commercial sources. The
complexes [PtMe(ppy)(SMe2)], ppy = deprotonated 2-phenyl-
dried under vacuum. Yield: 87 mg, 95%; m.p. = 153–155 °C
(decomp.). C47H36F3FeNO2P2Pt (1016.4): calcd C, 55.5; H, 3.57;
N, 1.38; found: C, 54.7; H, 3.71; N, 1.59%. NMR data in CDCl3:
1H NMR: δ = 3.95 (br s, CH dppf, 2H), 4.44 (br s, CH dppf, 2H),
4.48 (br s, CH dppf, 2H), 4.57 (br s, CH dppf, 2H), aromatic
2
1
protons: 6.5–8.0; 31P NMR: δ = 15.8 (d, J(PP) = 21.1, J(PtP) =
pyridine, 1a,47 [PtMe(bhq)(SMe2)], bhq = deprotonated benzo-
[h]quinoline, 1b,47 [Pt(p-MeC6H4)(ppy)(SMe2)], 1c,48 [Pt(p-MeC6H4)-
(bhq)(SMe2)], 1d,48 [PtMe(κ1C-ppy)(dppf)], dppf = 1,1′-bis-
(diphenylphosphino)ferrocene, 2a,49 [PtMe(κ1C-ppy)-(dppe)],
dppe = bis(diphenylphosphino)ethane, 2b,47 [Pt(p-MeC6H4)-
(κ1C-ppy)(dppe)], 2c,47 [Pt(p-MeC6H4)(κ1C-bhq)-(dppe)], 2d,47
[PtMe(ppy)(PPh3)], 6a,49 [Pt(p-MeC6H4)(ppy)-(PPh3)], 6b,50 and
[PtMe(tpy)(PPh3)], tpy = deprotonated 2-p-tolylpyridine, 6c,51
were made as reported previously.
2
1
4096 Hz, P trans to N, 1P), δ = 24.7 (d, J(PP) = 21.2, J(PtP) =
1959 Hz, P trans to C, 1P).
The following complexes were prepared similarly by using
the appropriate starting materials.
[Pt(ppy)(dppe)][CF3CO2], 4b
Yield: 79 mg, 92%; m.p.
=
128–131 °C (decomp.).
C39H32F3NO2P2Pt (860.4): calcd C, 54.4; H, 3.75; N, 1.63;
found: C, 53.7; H, 3.98; N, 1.79%. NMR data in CDCl3: 1H
NMR: δ = 2.61 (m, CH2 dppe, 4H), aromatic protons: 8.21 (t,
[Pt(ppy)(CF3CO2)(SMe2)], 3a
To a solution of complex [PtMe(ppy)(SMe2)], 1a, (42.6 mg, 3J(HH) = 4J(PH) = 4.8 Hz, 3J(PtH) = 27.0 Hz, H6 of ppy, 1H),
0.1 mmol) or [Pt(p-MeC6H4)(ppy)(SMe2)], 1c, (50.2 mg, other aromatic protons: 6.8–8.1; 31P NMR: δ = 41.4 (s, J(PtP) =
1
0.1 mmol) in dichloromethane (20 mL) was added a stock 3776 Hz, P trans to N, 1P), δ = 51.2 (s, 1J(PtP) = 1834 Hz, P trans
solution of CF3CO2H (781 μL of 1% solution of CF3CO2H in to C, 1P).
13370 | Dalton Trans., 2013, 42, 13369–13380
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[Pt(bhq)(dppf)][CF3CO2], 4c
6.1–8.1. 31P NMR 8b: 9.1 (s, 1J(PtP) = 4555 Hz, P trans to N, 1P).
1H NMR 7b: δ = 2.1 (s, Me of p-tolyl, 3H), aromatic protons:
9.43 (d, 3J(HH) = 4.5 Hz, H6 of κ1N-Hppy ligand, 1H), other aro-
Yield: 98 mg, 94%; m.p. = 184–186 °C (decomp.). C49H36F3Fe-
NO2P2Pt (1040.4): calcd C, 56.5; H, 3.49; N, 1.35; found:
C, 56.8; H, 3.67; N, 1.51%. NMR data in CDCl3: H NMR: δ =
3.96 (br s, CH dppf, 2H), 4.44 (br s, CH dppf, 2H), 4.52 (br s,
CH dppf, 2H), 4.56 (br s, CH dppf, 2H), aromatic protons: 8.20
1
1
matic protons: 6.1–8.1. 31P NMR 7b: 6.0 (s, J(PtP) = 5123 Hz,
P trans to O, 1P).
3
4
3
[PtMe(κ1N-Htpy)(PPh3)(CF3CO2)], 7c + 8c
(t, J(HH) = J(PH) = 4.5 Hz, J(PtH) = 22.5 Hz, H6 of bhq, 1H),
other aromatic protons: 6.8–8.5; 31P NMR: δ = 16.7 (d, J(PP) = Yield: 72.3 mg, 96%; m.p.
= 207–210 °C (decomp.).
2
1
2
20.1, J(PtP) = 4104 Hz, P trans to N, 1P), δ = 24.7 (d, J(PP) = C33H29F3NO2PPt (754.6): calcd C, 52.5; H, 3.87; N, 1.86; found:
20.3, 1J(PtP) = 2004 Hz, P trans to C, 1P).
C, 52.2; H, 3.57; N, 1.90%. NMR data in CDCl3: 1H NMR 8c: δ =
3
2
−0.23 (d, J(PH) = 3.4 Hz, J(PtH) = 80.6 Hz, Me, 3H), 2.51 (s,
Me of κ1N-Htpy, 3H) aromatic protons: 9.16 (m, 3J(PtH) =
34.1 Hz, H6 of κ1N-Htpy ligand, 1H), other aromatic protons:
6.8–8.4. 31P NMR 8c: 15.3 (s, 1J(PtP) = 4619 Hz, P trans to N,
[Pt(bhq)(dppe)][CF3CO2], 4d
Yield: 82 mg, 93%; m.p.
=
219–221 °C (decomp.).
C41H32F3NO2P2Pt (884.4): calcd C, 55.6; H, 3.65; N, 1.58;
found: C, 54.9; H, 3.64; N, 1.63%. NMR data in CDCl3: 1H
NMR: δ = 2.64 (m, CH2 dppe, 4H), aromatic protons: 8.48 (d,
1
3
1P). H NMR 7c: δ = 0.40 (d, J(PH) = 2.6 Hz, 2J(PtH) = 64.8 Hz,
Me, 3H), 2.52 (s, Me of Htpy, 3H), aromatic protons: 9.22 (d,
3J(HH) = 5.6 Hz, H6 of κ1N-Htpy ligand, 1H), other aromatic
protons: 6.8–8.4. 31P NMR 7c: 13.7 (s, 1J(PtP) = 5204 Hz, P trans
to O, 1P).
3
3J(HH) = 8.1 Hz, J(PtH) = 24.6 Hz, H6 of bhq, 1H), other aro-
matic protons: 7.1–8.1; 31P NMR: δ = 41.5 (s, J(PtP) = 3766 Hz,
1
P trans to N, 1P), δ = 51.3 (s, 1J(PtP) = 1877 Hz, P trans to
C, 1P).
Kinetic studies using 1H NMR monitoring
The 7c/8c isomerization process in CDCl3 or C6D6 was moni-
tored by 1H NMR spectroscopy as follows: a small sample
(10 mg) of [PtMe(tpy)(PPh3)], 6c, was dissolved in 0.75 mL of
CDCl3 (or C6D6) in a sealed NMR tube, and an equimolar
amount of acid was added. The NMR spectra were recorded
several times at room temperature over about 2 h until the
equilibrium was gradually formed in solution. A similar reac-
tion in the presence of 10 equiv. excess free Htpy reagent was
performed in CDCl3.
[Pt2(bhq)2(CF3CO2)2(μ-dppf)], 5
To a solution of [Pt(bhq)(CF3CO2)(SMe2)], 3b, (52.4 mg,
0.1 mmol) in acetone (20 mL) was added dppf (27.7 mg,
0.05 mmol). The mixture was stirred at room temperature for
1 h. The solvent was removed and the residue was triturated
with diethyl ether (2 × 2 mL). The product as a yellow solid was
dried under vacuum. Yield: 60 mg, 82%; m.p. = 214–216 °C
(decomp.). C64H44F6FeN2O4P2Pt2 (1526.6): calcd C, 50.3; H,
2.90; N, 1.83; found: C, 50.5; H, 2.76; N, 1.95%. NMR data in
1
CDCl3: H NMR: δ = 4.37 (br s, CH dppf, 4H), 4.79 (br s, CH
X-Ray structure determination
dppf, 4H), (aromatic protons): 6.5–9.0; 31P NMR: δ = 10.2 (s,
1J(PtP) = 4391 Hz, P trans to N, 2 P).
X-Ray diffraction data were collected at room temperature by
the ω-scan technique on Agilent Technologies four-circle diffr-
actometers: for 3b on a Super Nova (Atlas detector) with
mirror-monochromatized CuKα radiation (λ = 1.54178 Å), and
for the remaining two (4d, 7a) on an Xcalibur (Eos detector)
with multilayer optics monochromatized MoKα radiation (λ =
0.71073 Å). The data were corrected for Lorentz-polarization
and absorption effects.52 Accurate unit-cell parameters were
determined by a least-squares fit of 6081 (3b), 12 824 (4d), and
1069 (7a) reflections of highest intensity, chosen from the
whole experiment. The structures were solved with SIR9253
and refined with the full-matrix least-squares procedure on F2
by SHELXL97.54 Scattering factors incorporated in SHELXL97
were used. All non-hydrogen atoms were refined anisotropi-
cally, all hydrogen atoms were placed in the calculated posi-
tions, and refined as the ‘riding model’ with the isotropic
displacement parameters set at 1.2 (1.5 for methyl groups)
times the Ueq value for appropriate non-hydrogen atoms. Rele-
The following complex mixtures were prepared by CF3CO2H
reaction as described above for 3a, using the appropriate start-
ing materials [PtR(C^N)(PPh3)], 6: [PtMe(κ1N-Hppy)(PPh3)-
(CF3CO2)], 7a + 8a. Yield: 67.1 mg, 91%; m.p. = 200–202 °C
(decomp.). C32H27F3NO2PPt (740.6): calcd C, 51.9; H, 3.67;
N, 1.89; found: C, 52.0; H, 3.60; N, 2.20%. NMR data in CDCl3:
3
2
1H NMR 8a: δ = −0.20 (d, J(PH) = 3.2 Hz, J(PtH) = 80.7 Hz,
Me, 3H), aromatic protons: 9.18 (m, 3J(PtH) = 32.1 Hz, H6 of
κ1N-Hppy ligand, 1H), other aromatic protons: 6.8–8.4.
31P NMR 8a: 15.2 (s, 1J(PtP) = 4621 Hz, P trans to N, 1P).
1H NMR 7a: δ = 0.41 (d, 3J(PH) = 2.5 Hz, 2J(PtH) = 65.0 Hz,
Me, 3H), aromatic protons: 9.25 (d, 3J(HH) = 5.6 Hz, H6 of
κ1N-Hppy ligand, 1H), other aromatic protons: 6.8–8.4.
31P NMR 7a: 13.7 (s, 1JPtP = 5197 Hz, P trans to O, 1P).
[Pt(p-MeC6H4)(κ1N-Hppy)(PPh3)(CF3CO2)], 7b + 8b
Yield: 70.2 mg, 86%; m.p.
= 233–236 °C (decomp.). vant crystal data are listed in Table 1, together with refinement
C38H31F3NO2PPt (816.7): calcd C, 55.9; H, 3.83; N, 1.72; found: details.
1
C, 55.7; H, 3.60; N, 1.99%. NMR data in CDCl3: H NMR 8b:
Crystallographic data (excluding structure factors): CCDC
δ = 1.97 (s, Me of p-tolyl, 3H), aromatic protons: 9.21 (d, 3J(HH) = 919435 (3b), CCDC 919436 (4d), and CCDC 919437 (7a). The
7.1 Hz, H6 of κ1N-Hppy ligand, 1H), other aromatic protons: cif file for 7c, which is isostructural with 7a, has been
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Table 1 Crystal data and structure refinement for the complexes
3b
4d
7a
Formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
α (°)
β (°)
C
17H14F3NO2PtS
[C39H32NP2Pt]·C2F3O2
884.71
C32H27F3NO2PPt
740.6
548.44
Monoclinic
P21 (no. 4)
4.6705(1)
13.9784(3)
12.7548(2)
90
94.458(2)
90
830.19(3)
2
Monoclinic
P21/c (no. 14)
11.6274(2)
13.9851(2)
21.7088(3)
90
92.005(1)
90
3527.91(9)
4
Triclinic
ˉ
P1 (no. 2)
10.2693(17)
11.3130(7)
13.853(2)
87.140(8)
72.665(15)
70.061(9)
1441.8(4)
2
γ (°)
Vol/Å3
Z
Dcalcd [g cm−3
μ/mm−1
F (000)
]
2.19
17.39
520
1.67
4.12
1744
1.71
4.97
724
Data:
Collected
Unique (Rint
8410
2974 (0.062)
2800
0.034
0.036
0.093
0.095
1.11
0.96/−1.48
34 752
7775 (0.021)
6306
0.026
0.039
0.055
0.059
1.14
0.79/−0.45
10 415
5899 (0.046)
5365
0.028
0.033
0.070
0.073
1.03
1.57/−1.02
)
With I > 2σ(I)
R1 [I > 2σ(I)]
R1 [all data]
wR2 [I > 2σ(I)]
wR2 [all data]
S
Max/min Δρ (e Å−3
)
deposited with the CDB as a private communication, depo-
sition number 919438.
Results and discussion
Synthesis and reactions
The routes to prepare the bis-chelate complexes [Pt(C^N)(P^P)]-
[CF3CO2], 4, are described in Scheme 1. Thus, each of the
known starting complexes [PtR(C^N)(SMe2)], 1,47,48 in which
R = Me or p-MeC6H4 and C^N is either ppy (deprotonated
2-phenylpyridine) or bhq (deprotonated benzo[h]quinoline), was
reacted with 1 equiv. of CF3CO2H, yielding the complexes
[Pt(C^N)(CF3CO2)(SMe2)], 3. It is to be noted that in these reac-
tions the Pt–C bond of the Pt–R moiety, rather than that of the
chelating Pt(C^N) moiety, is cleaved and that a Pt–CF3CO2
bond is formed. Complexes 3 were reacted with 1 equiv. of
either of the diphosphine reagents dppe = bis(diphenylpho-
sphino)ethane or dppf = 1,1′-bis(diphenylphosphino)ferrocene
(P^P) to give the bis-chelate complexes [Pt(C^N)(P^P)]-
[CF3CO2], 4. Thus, in each case P^P at first displaces SMe2 and
then the CF3CO2 ligand is disposed from the coordination
sphere to form the cationic complexes 4.
Scheme 1
Besides, as has been reported previously, when the known is removed and subsequently the C^N chelate is formed by dis-
complexes 1 are reacted with 1 equiv. of P^P reagents dppe or posing the CF3CO2 ligand from the coordination sphere.
dppf,47,49 the P^P at first displaces SMe2 and then its stronger
Reactions of the complexes [Pt(C^N)(CF3CO2)(SMe2)], 3,
chelating ability (as compared with that of the C^N chelate) with P^P ligands (as described in Scheme 1) to give complexes
would force the C^N chelate to open up from the N site and [Pt(C^N)(P^P)][CF3CO2], 4, were investigated in more detail
complexes 2 are formed. Reaction of the complexes 2 with 1 and the results are depicted in Scheme 2. When the complex
equiv. of CF3CO2H was shown to be an alternative method for 3b is reacted with 0.5 equiv. of dppe, 0.5 equiv. of the related
the preparation of the complexes 4. Thus, the acid attacks the bis-chelate product, 4d, was formed along with 0.5 equiv.
R ligand of the complex and displaces it with CF3CO2 and R–H of the unreacted starting complex 3b. In contrast, when the
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2-p-tolylpyridine, were reacted with 1 equiv. of CF3CO2H, the
course of the reaction seemed different from those involving
complexes 1 in Scheme 1. Thus, the reaction gave [PtR-
(κ1N-HC^N)(PPh3)(CF3CO2)], as an equilibrium mixture of two
isomers 7 and 8, as will be discussed in detail in the following
section of kinetic investigations. When each one of the mix-
1
tures was finally obtained as a pure solid, the related H NMR
studies (see below) confirmed that the ratio of the two isomers
(see Scheme 3) is highly dependent on the R group, being 3 : 2
(when R = Me) and 1 : 20 (when R = p-MeC6H4).
Characterization of the new complexes
The new complexes 3, 4, 5, 7, and 8 were characterized using
1H NMR and 31P NMR (when applicable) spectroscopy,
elemental analysis, and single crystal X-ray crystallography
(typically for the complexes 3b, 4d, and 7a), and the data are
collected in the Experimental section.
Typically, in the 1H NMR spectrum of complex [Pt(bhq)-
(CF3CO2)(SMe2)], 3b, a singlet signal at δ 2.73, accompanied by
platinum satellites with 3J(PtH) = 54.5 Hz, was observed for the
terminal SMe2 ligand. This coupling constant is relatively
larger than the usually reported 3J(PtH) values for Pt–SMe2
locating trans to C, which is consistent with SMe2 being trans
to N with a lower trans influence than C.24 The other detect-
able peak was for the C–H proton adjacent to the N ligating
atom of bhq, i.e. H6, observed as a doublet at 8.73 (with 2J(HH) =
5.4 Hz) which is also coupled to the platinum center to give
satellites with 3J(PtH) = 35.1 Hz. The complexes 3 are air-stable
solids and complex [Pt(bhq)(CF3CO2)(SMe2)], 3b, was structu-
rally characterized (Fig. 1i). The complex has square planar
stereochemistry at platinum (maximum deviation from the
least-squares plane through four atoms is 0.008(7) Å), with the
greatest distortion associated with the constraints of the
chelate Pt(bhq) ring. The C–H⋯O intermolecular hydrogen
bonding, in which the C–H is from one of the Me groups on
the SMe2 ligand of a molecule and O is the uncoordinated
oxygen atom of the acetate ligand from another molecule, is
also shown (Fig. 1ii).
Scheme 2
complex 3b was reacted with 0.5 equiv. of dppf, then the new
dimeric complex [Pt2(bhq)2(CF3CO2)2(μ-dppf)], 5, was obtained
in a pure form. The latter reacted with 0.5 equiv. of dppf to
give 4c. Reversibly, when 4c was reacted with one equivalent
of the starting complex, 3b, the bimolecular complex 5 was
regenerated.
As is described in Scheme 2, reaction of the complex
[Pt(bhq)(CF3CO2)(SMe2)], 3b, with 1 equiv. of dppf gave the
bis-chelate complex [Pt(bhq)(dppf)][CF3CO2], 4c. It therefore
seems that the chelating ability of dppf has forced the CF3CO2
ligand to leave the coordination site. This is to be compared
with a similar reaction involving the analogous chloride
complex [Pt(C^N)(Cl)(DMSO)], which, on treatment with 1
equiv. of dppf, results in the formation of binuclear complex
[Pt2(C^N)2(Cl)2(μ-dppf)].55 This discrepancy is consistent with
the stronger coordinating ability of Cl in comparison with that
of CF3CO2.56
As typical examples of bis-chelate complexes 4, the com-
plexes 4a and 4d are considered for spectroscopic characteris-
ation. In the 31P NMR spectrum of [Pt(ppy)(dppf)][CF3CO2],
4a, the two different P atoms appeared as two doublet signals
flanked by Pt satellites at δ = 15.8, with 2J(PP) = 21 and 1J(PtP) =
4096 Hz, for the P trans to N, and δ = 24.7 for the P trans to C,
As is described in Scheme 3, when the known PPh3 ana-
logues of the SMe2 complexes 1, i.e. the complexes [PtR(C^N)-
(PPh3)], 6,51 in which C^N is ppy or tpy = deprotonated
2
1
with J(PP) = 21 and a much lower value of J(PtP) = 1959 Hz
due to the trans influence of C being much greater than that of
N. The observation of a significant PP coupling of 21 Hz in the
chelating dppf complexes 4a and 4c (see the Experimental
section) is not usual in the related complexes containing other
types of bisphosphine chelates such as dppe.47 Thus, no such
PP coupling has been resolved in the 31P NMR spectra of com-
plexes 4d (see below) and 4b (see the Experimental section) in
each of which a dppe ligand acts as a chelate. We attribute this
to a significantly larger P–Pt–P angle, as observed in the
reported square planar complexes (see the reported data for
dppf chelating complex 2a,49 with the P–Pt–P angle of 97.61°),
Scheme 3
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Fig. 1 (i) Thermal-ellipsoid representation and (ii) C–H⋯O intermolecular hydrogen bonding for complex [Pt(bhq)(CF3CO2)(SMe2)], 3b. Ellipsoids are drawn at the
50% probability level, hydrogen atoms are shown as spheres of arbitrary radius. Selected geometrical parameters (Å, °): Pt1–O4 2.113(5); Pt1–C11 1.972(8); Pt1–
N22 2.062(8); Pt1–S1 2.263(3); O(4)–C(5) 1.265(17); O(6)–C(5) 1.223(15); N22–Pt1–O4 94.9(5); N22–Pt1–C11 82.0(6); O4–Pt1–S1 91.6(4); C11–Pt1–S1 91.5(5); S(1)–
Pt(1)–N(22) 173.4(2); O(4)–Pt(1)–C(11) 176.8(8). Intermolecular hydrogen bond for the complex [Å and °]: D–H⋯A being C3–H3A⋯O6 (1 − x, −1/2 + y, 2 − z),
d(D–H) = 0.96, d(H⋯A) = 2.40, d(D⋯A) = 3.326(17), and ∠(DHA) = 161.
as compared to other cases with the angle being less than 90° 1J(PtP) confirms that each P atom is coordinated to a Pt atom
(84.53° in the dppe complex 4d, see below). In the 1H NMR in a trans disposition to the coordinating N atom of the pyri-
spectrum of [Pt(ppy)(dppf)][CF3CO2], 4a, 4 broad singlets were dine ring of the bhq ligand. Each CF3CO2 ligand is thus
observed for the α and β protons of the dppf ligand at δ = 3.95, located trans to the coordinated C atom of the bhq ligand. The
4.44, 4.48, and 4.57 confirming that the two Cp moieties of the four equivalent α protons and four equivalent β protons of the
dppf ligand are inequivalent. In the 31P NMR spectrum of cyclopentadienyl rings appear as two broad singlets at δ 4.79
[Pt(bhq)(dppe)][CF3CO2], 4d, the two different P atoms appear and 4.37.
as two singlet signals at δ = 41.5, for the P trans to N (with
The complex mixtures 7 + 8 were also characterized by their
1J(PtP) = 3766 Hz), and δ = 51.3, for the P trans to C (with a 1H and 31P NMR spectra. Typically, in the 31P NMR spectrum
1
much lower value of J(PtP) = 1877 Hz due to the trans influ- of [PtMe(κ1N-Hppy)(PPh3)(CF3CO2)], 7a + 8a, a singlet signal at
1
ence of C being much greater than that of N). In the H NMR δ = 13.7, with 1J(PtP) = 5197 Hz, is attributed to the P atom
spectrum of 4d, CH2 protons of the dppe chelating ligand are trans to the O ligating atom of the CF3CO2 ligand for isomer
observed as a multiplet at δ = 2.64. A doublet signal at δ = 8.48 7a, while the singlet signal at δ = 15.2, which is coupled to Pt
(with 3J(HH) = 8.1 Hz), which is coupled to the platinum with a significantly lower value of 1J(PtP) = 4621 Hz, is
3
center to give satellites with J(PtH) = 24.6 Hz, is attributed to assigned to the P atom trans to the N ligating atom of the
the C–H proton adjacent to the N ligating atom of bhq, i.e. H6.
κ1N-Hppy ligand for isomer 8a; notice that the trans influence
The complexes 4 are air-stable solids and complex [Pt(bhq)- of N is considerably greater than that of the O ligating atom of
1
(dppe)][CF3CO2], 4d, was structurally characterized (Fig. 2i). CF3CO2. Consistently, in the H NMR spectrum of the mixture
The complex has square planar stereochemistry at platinum 7a + 8a, the doublet signal expected for the Me ligand of
(maximum deviation from the least-squares plane through isomer 8a is observed at δ = −0.20 (with 3J(PH) = 3.2 Hz)
2
four atoms is 0.083(1) Å), with the greatest distortions associ- which is also coupled to the Pt center with J(PtH) = 80.7 Hz
ated with the constraints of the chelating Pt(bhq) and P2Pt (Me trans to O), while that for the isomer 7a, appearing at δ =
rings. The two different kinds of C–H⋯O intermolecular 0.41 (with 3J(PH) = 2.5 Hz), couples to the Pt center with a
hydrogen bonding involving the two oxygen atoms of the significantly smaller value of 2J(PtH) = 65.0 Hz, for the Me
CF3CO2 counter anion, are also shown (Fig. 2ii). As a result of ligand trans to N.
this the two C–O bonds, C(2A)–O(1A) 1.2159 and C(2A)–(O2A)
1.2261, are of almost the same length.
The complex mixtures 7 + 8 are air-stable solids and crystals
of the complex [PtMe(κ1N-Hppy)(PPh3)(CF3CO2)], isomer 7a
In the 31P NMR spectrum of the dimeric complex (the isomer with CF3CO2 trans to PPh3), were formed from crys-
[Pt2(bhq)2(CF3CO2)2(μ-dppf)], 5, the two equivalent P atoms tallization attempts for the complex mixture 7a + 8a. The
appear as a singlet signal at δ 10.2, coupled to Pt atoms to give complex has square-planar stereochemistry at platinum, with
satellites with 1J(PtP) = 4391 Hz. The equivalency of the the greatest distortion being due to a rather large steric effect
P atoms suggests that dppf is acting as a spacer ligand between between PPh3 and κ1N-Hppy ligands (Fig. 3i). The intermole-
the two Pt(bhq)(CF3CO2) moieties, and the large value of cular C–H⋯F hydrogen bonding is also shown.
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Fig. 2 (i) Thermal-ellipsoid representation and (ii) two different kinds of C–H⋯O intermolecular hydrogen bonding for complex [Pt(bhq)(dppe)][CF3CO2], 4d. Ellip-
soids are drawn at the 50% probability level, hydrogen atoms are shown as spheres of arbitrary radius. Selected geometrical parameters (Å, °): Pt1–P1 2.2540(8);
Pt1–P2 2.3197(8); Pt1–N27 2.104(3); Pt1–C38 2.077(3); N27–Pt1–C38 80.18(12); P1–Pt1–P2 84.53(3); P1–Pt1–C38 96.10(9); P2–Pt1–N27 99.47(8); P1–Pt1–N27
173.85(8); P2–Pt1–C38 176.66(8). Intermolecular hydrogen bonds for the complex [Å and °]: D–H⋯A are C35–H35⋯O2A (1 + x, 1/2 − y, −1/2 + z) [with d(D–H) =
0.93, d(H⋯A) = 2.45, d(D⋯A) = 3.333(5), and ∠(DHA) = 159] and C11–H11⋯O1A (x, y, −1 + z) [with d(D–H) = 0.93, d(H⋯A) = 2.42, d(D⋯A) = 3.314(5), and
∠(DHA) = 162].
Fig. 3 (i) Thermal-ellipsoid representation and (ii) intermolecular C–H⋯F hydrogen bonding for complex [PtMe(κ1N-Hppy)(PPh3)(CF3CO2)], isomer 7a. Ellipsoids are
drawn at the 50% probability level, hydrogen atoms are shown as spheres of arbitrary radius. Selected geometrical parameters (Å, °): Pt1–P1 2.1815(11); Pt1–N19
2.146(3); Pt1–O31 2.121(3); Pt1–C38 2.058(5); O31–C32 1.244(5); O33–C32 1.201(6); O31–Pt1–N19 86.64(13); O31–Pt1–C38 87.75(16); P1–Pt1–C38 89.56(14); P1–
Pt1–N19 96.09(9); N19–Pt1–C38 174.03(16); P1–Pt1–O31 177.13(9). Intermolecular hydrogen bond for the complex [Å and °]: D–H⋯A being C4–H4⋯F37 (−x, −y, 1
− z), d(D–H)= 0.93, d(H⋯A) = 2.47, d(D⋯A) = 3.389(7), and ∠(DHA) = 169.
Investigation of the product isomerization
the isomerization of 7c to 8c is proposed to occur by a disso-
ciative mechanism involving dissociation of the neutral
As described above (relating to the reactions in Scheme 3), κ1N-Htpy ligand.
when, for example, the complex [PtMe(tpy)(PPh3)], 6c, was
The isomerization process involving products of the reac-
reacted with 1 equiv. of CF3CO2H, the reaction gave [PtMe- tion of complex [PtMe(tpy)(PPh3)], 6c, with 1 equiv. of
(κ1N-Htpy)(PPh3)(CF3CO2)] as an equilibrium mixture of two CF3CO2H, i.e. formation of an equilibrium mixture containing
isomers 7c (in which the CF3CO2 ligand is trans to PPh3) and isomers 7c and 8c, was monitored by using 1H NMR spec-
8c (in which CF3CO2 is trans to Me). On the basis of kinetic troscopy. Changes in the spectrum during the process in
studies (using 1H NMR monitoring), as shown in Scheme 4, CDCl3 solvent are shown in Fig. 4. Thus, immediately after
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Scheme 4
Fig. 5 Changes in molar concentration versus time during equilibrium for-
mation involving the isomerization process of 7c to 8c in CDCl3 as monitored by
1H NMR spectroscopy at 25 °C. The upper curve shows the decrease in concen-
tration of isomer 7c and the lower curve shows the increase in concentration of
8c.
in which:
ke ¼ k1 þ k
ꢀ1
ꢀ1
k1 ¼ ke=ð1 þ Keq
Þ
k
¼ ke=ð1 þ KeqÞ
ꢀ1
Fig. 4 Monitoring the 7c/8c isomerization process by 1H NMR spectroscopy (in
the aliphatic region) in CDCl3 at 25 °C. The first spectrum (0 min) is obtained
immediately after the addition of acid to the starting complex [PtMe(tpy)(PPh3)],
6c, in 1 : 1 molar ratio.
In these equations, Keq = equilibrium constant (calculated as
the relative integrations of peaks due to the Pt–Me moieties of
7c/8c in the final 1H NMR spectrum obtained after 2 h, see
Fig. 4), and ke = rate of equilibrium formation. Thus, the calcu-
lated values are Keq = 0.730, ke = 2.27 × 10−3 s−1, k1 = 0.96 ×
10−3 −1, and k−1 = 1.31 × 10−3 −1. Notice that when the same
s s
addition of the acid to a solution of 6c, the signals (see spec-
trum at 0 min) due to starting complex 6c completely dis-
appeared and those for the isomer 7c (with a doublet for the
Me ligand at δ = 0.40 and a singlet for the Me of the κ1N-Htpy
ligand at δ = 2.52) appeared along with comparatively short
signals assigned to a small amount of the isomer 8c (with a
doublet for the Me ligand at δ = −0.23 and a singlet for the Me
of the κ1N-Htpy ligand at δ = 2.51); the ratio 7c : 8c at this point
is 7 : 1. As time passes, the concentration of 7c decreases while
that of 8c increases until equilibrium is reached after nearly
30 min and a 3 : 2 ratio for 7c : 8c is obtained.
isomerization process is performed in the presence of 10
equiv. of excess free Htpy reagent, then ke is significantly
decreased to 0.56 × 10−3 s−1 and the ratio 7c : 8c after nearly
30 min is significantly increased to 5 : 2 (see Table 2 for the
complete data). This is in agreement with the mechanism
shown in Scheme 4 in which the 7c → 8c isomerization
process involves prior dissociation of the κ1N-Htpy ligand as a
rate determining step. It is also worth mentioning that we did
not attempt to follow the same process in the presence of
excess PPh3 reagent in order to decline the involvement of the
reagent in the dissociation step, as we found that immediately
after the addition of the reagent, the related 1H NMR spectrum
became very complicated, probably due to displacement of the
other ligands.
Variation of molar concentration versus time during equili-
brium formation involving the isomerization process of 7c to
1
8c in CDCl3, as monitored by H NMR spectroscopy, is shown
in Fig. 5.
The data were fitted by eqn (1):57
The same isomerization process 7b → 8b occurring after
CF3CO2H treatment of the complex [Pt(p-MeC6H4)(ppy)-
(PPh3)], 6b, gave a product ratio 7b : 8b = 1 : 20, meaning that
the nature of the R group (Me in complex mixtures 7a/8a or
At ¼ Ae þ ðA0 ꢀ AeÞexpðꢀketÞ
ð1Þ
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Table 2 Dataa for the equilibrium formation between isomers 7c and 8c at 25 °C
Solvent
k1 (s−1) × 103
k−1 (s−1) × 103
ke (s−1) × 103
Keq
Ratio 7c : 8c at the start of process
Product ratio 7c : 8c after 30 min
CDCl3
C6D6
0.96 (0.24)b
2.43
1.31 (0.32)b
2.43
2.27 (0.56)b
4.86
0.73 (0.73)b
1.00
7 : 1 (7 : 1)b
3 : 1
3 : 2 (5 : 2)b
1 : 1
a Keq = equilibrium constant and ke = rate of equilibrium formation. b Values for the reaction in the presence of 10 equiv. excess free Htpy
reagent.
Fig. 6 Monitoring of the 7c/8c isomerization process by 1H NMR spectroscopy
(in the aliphatic region) in C6D6 at 25 °C. The first spectrum (0 min) is obtained
immediately after the addition of acid to the starting complex [PtMe(tpy)(PPh3)],
6c, in 1 : 1 molar ratio. Peaks due to the Me of the κ1N-Htpy ligand for both
isomers are overlapping.
Fig. 7 Changes in molar concentration versus time during equilibrium for-
mation in the isomerization process of 7c to 8c in C6D6 as monitored by
1H NMR spectroscopy at 25 °C. The upper curve shows the decrease in the
concentration of isomer 7c and the lower curve shows the increase in the con-
7c/8c, and p-MeC6H4 in complex mixture 7b/8b) has a tremen-
dous effect on the tendency of isomerization process 7 → 8.
centration of 8c.
Similar investigations were performed for the same process
1
in benzene-d6 solution and the related H NMR spectroscopy
trifluoroacetate ligand. We exclude this possibility because, in
monitoring spectra and the variation of molar concentration
contrast with our observation (see above) in which addition of
versus time during equilibrium formation are shown in Fig. 6
and 7, respectively. Immediately after addition of the acid to a
solution of 6c, the signals (see spectrum at 0 min) due to start-
ing complex 6c completely disappeared and those for the
free HC^N reagent significantly decreased the reaction rate, in
the latter case the addition of excess free HC^N reagent
(having a nitrogen donor) is expected to increase the rate of
the process as compared to the original case in which a
isomers 7c and 8c appeared with the ratio 7c : 8c being 3 : 1.
pendant O atom (having a poorer donor ability compared to
When the equilibrium was reached after nearly 30 min, the
the softer N donor) is forming the transition state.
ratio for 7c : 8c had changed to 1 : 1 and the ke was calculated
to be 4.86 × 10−3 s−1 (see Table 2).
Therefore the rate for equilibrium formation between
isomers 7c and 8c is more than two times faster in C6D6 than
Conclusions
in CDCl3 (see Table 2). These results comply well with the In reactions of the p-tolyl- (or methyl)-cycloplatinated(II) com-
leaving group in the rate determining step (see Scheme 4) plexes with 1 equiv. of CF3CO2H, studied in the present work,
being the neutral ligand κ1N-Htpy rather than the anionic we found that the selectivity in M–C bond cleavage depends
−
group CF3CO2 (for which the resulting ionic intermediate on the ancillary ligands on the complex. Three types of com-
[PtMe(κ1N-Htpy)(PPh3)]+(CF3CO2)−, formed in the rate deter- plexes [PtR(C^N)(SMe2)], 1, [PtR(κ1C-C^N)(P^P)], 2, and [PtR-
mining step is expected to be more stable in the more polar (C^N)(PPh3)], 6, were subjected to the acid reactions. Thus,
solvent CDCl3 than in non-polar solvent C6D6).
while during the acid reactions involving complexes 1, the Pt–R
One may argue about the possibility of an associative mech- bond is cleaved and the complexes [Pt(C^N)(CF3CO2)(SMe2)],
anism for the isomerization process 7 → 8, through a tran- 3, are formed, a different behaviour was observed in the
sition state involving formation of a bridge between two related reaction involving the analogous PPh3 complexes 6.
metallic centres via, for example, the pendant O atom of the During the latter reaction, the Pt–C bond between Pt and the
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C^N chelate, rather than that between Pt and the R ligand, is different. Here, the two kinds of “soft” Pt–C(aryl) bonds are
broken and the complexes [PtR(κ1N-C^N)(PPh3)(CF3CO2)], as located trans to “soft” P donors and we suggest that the driving
an equilibrium mixture of two isomers 7 and 8, are formed. force for protonolysis of the Pt–C bond of Pt–R over the Pt–C
We used the conjugation of the trans influence, the concept of (aryl) bond of κ1C-C^N may be due to formation of the inter-
antisymbiosis (put forward by Pearson58) and the hard/soft mediate E, with the strong ligating ability of the pendant N
nature of the donor atoms of the ligands to explain these site- atom and the strong chelating appetite of the C^N bidentate
selective protonation and the isomerization processes and our ligand stabilizing it as a six coordinated cationic complex;
suggestions are depicted in Scheme 5. Upon protonation of through E, the final reductive elimination occurs, preferably
the isostructural square-planar complexes 1 or the analogous via R–H over that involving the Pt–C(aryl) bond of the
PPh3 complexes 6, the related complex D is formed, followed C^N chelate, and the bis-chelate complexes [Pt(C^N)(P^P)]-
by C–H bond reductive elimination.59 In complexes 6, the (CF3CO2), 4, are formed.
“soft” Pt–C(aryl) bond of the C^N chelate is trans to a “soft”
After protonation of each of the complexes [PtR(C^N)-
PPh3 ligand and this would labilize the Pt–C bond, i.e. they (PPh3)], 6, the related kinetic product complex 7 (with PPh3
show antisymbiotic behaviour, in comparison with the Pt–C being trans to the CF3CO2 ligand) is yielded, which seems to
bond of the Pt–R moiety which is located trans to the N donor be in equilibrium with the corresponding isomeric form 8
ligand that is significantly harder than P donors. In this cir- (with PPh3 being trans to the κ1N-C^N ligand), and the equili-
cumstance, the Pt–C(aryl) bond of the C^N chelate is more brium between 7 and 8, suggested to occur through a dissocia-
prone to be involved in C–H reductive elimination as com- tive mechanism, is reached after about 30 min. We believe that
pared to the Pt–C of Pt–R. In the related case of CF3CO2H reac- the antisymbiotic effect can help in understanding this
tion involving the analogous SMe2 complexes 1, an opposing process by considering that the softest ligand (C-donor) has a
effect is observed. Although both PPh3 and SMe2 are con- tendency to be located trans to the hardest ligand (O-donor).
sidered as soft bases,58 the PPh3 ligand in the complexes 6 This tendency is significantly augmented when R = p-MeC6H4,
exerts a greater trans influence as compared with the SMe2 i.e. in the equilibrium between 7b and 8b with the product
ligand in the analogous complexes 1. Besides, we understand ratio 7b : 8b being 1 : 20 in chloroform solvent, as compared
that both the S atom donor of the SMe2 ligand and the N with the case of R = Me, i.e. in the equilibrium between 7a and
donor atoms of pyridine derivative ligands have similar trans- 8a or 7c and 8c in which the product ratio 7a : 8a or 7c : 8c in
influences, but they exert a lower trans-influence when com- the same solvent is found to be 3 : 2. The results comply well
pared with the P donor atom of the phosphine ligand.60 The with the C(aryl) ligating atom in the isomeric pair 7b/8b being
Pt–C bonds in each of the complexes 1 are located either trans significantly softer than the C(methyl) ligating atom in the iso-
to a donor atom S or a donor atom N with similar trans-influ- meric pair 7a/8a or 7c/8c. In the same line of reasoning, in the
ences, and therefore we believe that the strong chelate effect of protonation process involving the analogous SMe2 complexes
the C^N bidentate ligand in the intermediate D is responsible [PtR(C^N)(SMe2)], 1, R–H is reductively eliminated (see
for the preference for protonolysis of the Pt–C bond of Pt–R Scheme 5) and the resulting kinetic product [Pt(C^N)(CF3CO2)-
over the Pt–C(aryl) bond of the C^N chelate. The situation (SMe2)] (not detected) in which the very soft C(aryl) ligating
for protonolysis of the complexes [PtMe(κ1C-C^N)(P^P)], 2, is atom of the C^N chelate is trans to SMe2, is instantaneously
Scheme 5
13378 | Dalton Trans., 2013, 42, 13369–13380
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converted to the thermodynamic product 3 in which the 19 T. Okada, I. M. El-Mehasseb, M. Kodaka, T. Tomohiro,
corresponding C(aryl) ligating atom is best stabilized by locat-
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13380 | Dalton Trans., 2013, 42, 13369–13380
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